1.前提

前面说到sendMessage携带的delay会被加上SystemClock.uptimeMillis() ，最终赋值给Message的when。

msg.when = SystemClock.uptimeMillis() + delayMillis;

那么when除了用来在链表里面作为排序依据以外，还在哪里用到了呢？

2.Looper.loop

首先看下Looper的loop里面，循环取queue的msg，如果msg为null，返回；否则执行msg

public final class Looper {public static void loop() {final Looper me = myLooper();final MessageQueue queue = me.mQueue;for (;;) {Message msg = queue.next(); // might block  获得下一个消息if (msg == null) {// No message indicates that the message queue is quitting.return;}msg.target.dispatchMessage(msg); //执行Messagemsg.recycleUnchecked(); //msg放入缓存}}
}

3.MessageQueue.next

看下queue.next()的实现，会通过when计算下一条消息需要等待的时长。

nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); 

public final class MessageQueue {Message next() {// Return here if the message loop has already quit and been disposed.// This can happen if the application tries to restart a looper after quit// which is not supported.final long ptr = mPtr;if (ptr == 0) {return null;}int pendingIdleHandlerCount = -1; // -1 only during first iterationint nextPollTimeoutMillis = 0; //下一条消息等待的时长for (;;) {if (nextPollTimeoutMillis != 0) {Binder.flushPendingCommands();}//等待nextPollTimeoutMillis，不往下执行nativePollOnce(ptr, nextPollTimeoutMillis);//时间到了synchronized (this) {// Try to retrieve the next message.  Return if found.final long now = SystemClock.uptimeMillis();Message prevMsg = null;Message msg = mMessages;if (msg != null) {if (now < msg.when) { //当前时间小于when// Next message is not ready.  Set a timeout to wake up when it is ready.nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); //等待时长} else {// Got a message.mBlocked = false;if (prevMsg != null) {prevMsg.next = msg.next;//将msg从链表中移除} else {mMessages = msg.next; //更新表头}msg.next = null;msg.markInUse();return msg;}} else {// No more messages.nextPollTimeoutMillis = -1;}                }// Run the idle handlers.// We only ever reach this code block during the first iteration.for (int i = 0; i < pendingIdleHandlerCount; i++) {final IdleHandler idler = mPendingIdleHandlers[i];mPendingIdleHandlers[i] = null; // release the reference to the handlerboolean keep = false;try {keep = idler.queueIdle();} catch (Throwable t) {Log.wtf(TAG, "IdleHandler threw exception", t);}if (!keep) {synchronized (this) {mIdleHandlers.remove(idler);}}}// Reset the idle handler count to 0 so we do not run them again.pendingIdleHandlerCount = 0;// While calling an idle handler, a new message could have been delivered// so go back and look again for a pending message without waiting.nextPollTimeoutMillis = 0;}}
}